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Is there evidence of space surrounding the BB singularity?

  1. Aug 31, 2013 #1
    Is there evidence of space surrounding the BB singularity?
     
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  3. Aug 31, 2013 #2

    mfb

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    I don't think this question makes sense. What does "surrounding" mean, if it does not refer to space in our universe (which did exist after the singulary, if there was any singularity at all)?
     
  4. Aug 31, 2013 #3
    At the BB did our universe include anything other than the singularity?
     
  5. Aug 31, 2013 #4

    marcus

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    "Singularity" means failure of a man-made mathematical model, where the model (not Nature) breaks down gives meaningless numbers or blows up.

    In physics history (going back over 100 years) there have been a bunch of singularities in various models (model of hydrogen atom, model of thermal radiation from a warm or hot object). They have been taken as symptoms that the math needed improvement, and they have been fixed. A singularity does not have to occur at a single point. A theory can suffer from this type of breakdown in a region of infinite volume, or on a surface of finite area etc etc. It does not mean "single point".

    What the public gets told has so far been a different story. It could be that general audience likes to be told about wondrous mysteries like a "point of infinite density". Maybe science journalists have done a poor job communicating so far. Maybe people like to believe stuff that stretches their credulity. Myths have always been popular.

    But among professional cosmologists (google "a tale of two big bangs" for an outreach article from Albert Einstein Institute) the cosmological singularity is used as TIME-MARKER, a shorthand for "the moment in time where our current cosmic model utterly blows up."
    The cosmic singularity is also taken not as a real event but as a SYMPTOM that the classical 1915 math that cosmic model is based on fails at high density and needs fixing, or replacement.

    So a number of people have been working on that and they have developed several quantized versions which do NOT blow up at the start of expansion. And so at this point the game is to TEST to see which of the alternative (non-singular) candidates gives the most accurate predictions. Observational data is scheduled to be reported in 2014 by the European Space Agency (ESA) which may help to decide which (non-singular) theory looks like a winner.

    Historically, "singularity" referred to oddity, eccentricity, something irregular or out of the ordinary. That is one of its senses, in English, and that is the origin of the mathematical usage of the term. It does not mean single point surrounded by empty space!
    That mental image (suggested by the sound of the word, not the real sense of the word) is totally on the wrong track.


    The difficulty with this question is that in professional science one does not assume that the real universe ever contained a singularity.
    The AEI outreach article (google "a tale of two big bangs") puts it succinctly near the bottom of the page. Most professional cosmologists would be quite surprised if it actually turned out that at the start of the current expansion the universe contained "a point of infinite density" (whatever that might mean.)

    You should probably rephrase your question to be about one or more of the candidate theories that actually say something about what the universe was like at the beginning of expansion.

    The model based on classical 1915 GR does not make any clear statement, it leaves the very beginning undescribed. In the most commonly used cosmic model space has infinite volume, with approximately uniform energy density growing without limit as you go back in time. So as you push the model back in time towards the start of expansion it simply becomes invalid, inapplicable, unreliable (near the start of expansion.) If you are interested in those very very early moments then you have to shift over to other models (eg. quantum cosmology) which are post-1915 based and where the energy density does NOT grow without bound. At very high energy density (approx uniform throughout space) quantum corrections can even make gravity REPEL rather than attract. So there can be a BOUNCE when very high energy densities are reached, in some of the models being studied.

    People have various ideas about the OVERALL SIZE of space at the beginning of expansion, i.e. the moment of highest energy density. Space might have been infinite volume (approx unif. energy density) or it might have been some finite volume (again with nearly uniform density). Or the picture might be complicated---highly uneven--- with different patches at different density exanding at different rates and only OUR part, that the standard cosmic models cover, being uniform enough so you can talk about a moment of highest density and a definite beginning of expansion.

    Personally I don't worry about the highly uneven pictures because what we can see is highly uniform (the CMB is same temperature in all directions up to one part in 100,000. Uniform to within 1/1000 of one percent. so I figure what we live in and what we have to study and try explaining is remarkably uniform and coordinated back in very early times, and can be well-approximated with uniform models. So why speculate about a totally inhomogeneous fantasy out beyond that we see no evidence of? It's interesting enough to focus on understanding what we live in without making up more complicated stuff.

    And even that we have no certainty about the overall size. The portion we actually observe changes over time as more light comes in from farther away, so the "observable" size is a very poor indicator of the actual size of what has to be modeled.
     
    Last edited: Aug 31, 2013
  6. Aug 31, 2013 #5
    Thanks Marcus. Another question - How large was this "singularity" if that is what it shall be called? It seems to me that this universe might have started out infinitely large or at least extremely large.
     
  7. Aug 31, 2013 #6

    marcus

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    Hi Keepit, I was editing post#4 around the same time you posted and did not see your post#5. But as it happens I was addressing just that question, about size. Not much can be said, given today's knowledge, about the overall size. Especially if you mean overall size "right at the moment expansion started" or at the moment of maximum density---which forces us to shift over to one of the quantum cosmology (QC) approaches that people are studying but which are still unproven.

    There is an energy density unit called Planck density. You can google "Planck units" or "planck density".

    According to one QC model, called Loop qc, a bounce occurs at 41% of Planck density. A young Loop researcher (Ed Wilson-Ewing) has challenged that and given an argument that the LQC bounce would occur at several orders of magnitude lower density if the effect of matter and radiation were included. But we could take 0.41 Planck as a kind of benchmark and calculate how small a volume the currently observable universe would have to be compressed down into in order to have 0.41 Planck energy density. Some absurdly small volume. Does such a speculative exercise really make sense, given the current state of knowledge?
    And that is only the OBSERVABLE portion, the galaxies and gas clouds and primordial stuff that we can currently SEE or could be getting signals from. That is not a fair estimate of the mass-energy content overall. You get a larger volume if you don't restrict to the region that is in principle observable by us today. But we don't know the overall size. so it involves mucho guesswork.
     
  8. Aug 31, 2013 #7

    marcus

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    I certainly agree that it could have. I should emphasize that I'm just a cosmology watcher from the sidelines. I love the subject and follow parts of the research literature, but don't speak with any authority. I think (or hope) that over the course of the next few years we will have some LOWER BOUND estimates of the size of the overall universe, based on measurements of the average curvature (which is very nearly zero but might turn out to be slightly positive.)

    A slight positive mean curvature would indicate a finite spatial volume. Analogous to a sphere. The sphere surface has a positive curvature and so it closes around on itself and is finite. If you think of the 2D surface of a sphere and then try to imagine the experience of living in the 3D analog of that.

    It may be possible to make a plausible case that the overall universe is at least such and such big, because if it were any smaller it would need to have more curvature than we see.

    And that lower bound would only apply in the finite case, it might still be infinite.

    I don't like the term "mind-boggling". But these discussion of size of universe, and also size of observable portion back at start of expansion (extremely small, comparable to hydrogen nucleus or hydrogen atom) are boggling. One gets into trying to imagine stuff that is too big or too small to comfortably imagine. I instinctively want to wait until the cosmologists have made a bit more progress on some of these questions and maybe it won't boggle me so much.

    You may be able to get more satisfactory comments from some other people at the forum, if I chicken out. :biggrin:
     
  9. Aug 31, 2013 #8
    "at very high energy densities (approx uniform throughout space), quantum corrections can even make gravity repel".
    Sorry to focus here on one small part of all that you said.
    Anyhow, are you saying that these quantum corrections may have taken place at the start of the BB. (I hope I didn't miss your point here. I'm just an amateur.
     
  10. Aug 31, 2013 #9

    marcus

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    I basically an amateur also, as a retired guy who has been avidly interested in the stuff for around 10 years, not an expert or authority.

    The answer to your question is yes--quantum corrections to be expect precisely at Big Bounce due to very high energy density. Conditions around start of expansion are exactly when you would expect quantum effects to be dominant. As I said there are SEVERAL COMPETING ideas of what could really have happened at the time the classical (non-quantum) model breaks down and suffers a singularity.

    One of the rival proposals is Loop quantum cosmology, based on LQG (the G could as well stand for geometry as gravity since the gravitational field is traditionally the field defining the shape/curvature of space and hence gravity=geometry).

    There are various ideas about the right way to make a quantum theory out of our classical theory of geometry (general rel). LQG is one approach to quantizing GR, that is quantizing geometry.
    It, or something like it, could turn out to be right. Whether right or wrong it is getting a lot of attention from researchers nowadays. And the way it works out with the LQC model of the start of expansion is that it is a BOUNCE.

    Essentially because nature resists being pinned down or crowded too much. So quantum effects or quantum corrections start to get important at extreme density or when you force the geometry into extreme curvature or extreme confinement. I'm not sure how to say this in words. If you follow back in time using the LQC version of quantized GR you find there was a bounce.

    The model says that there was a classical universe AFAWK like ours in its basic physics, that collapsed in a crunch and instead of a "singularity" when it reached an extremely high density like 1% of planck the quantum effects began being felt and slowed down the contraction, and at some point like 40% planck density it turned around and rebounded.

    So that is how that particular theory LQG (through its application to cosmology called LQC) describes the start of the expansion of our universe.

    Here's a keyword search to ALL the quantum cosmology research (not only the Loop part of it, but all kinds of QC) from 2009 to now. You can get some idea by scanning over the titles of the first 25 or 50 articles. They are mostly all free to look at on the arxiv.org website. Many have to do with bounce and the question of how we might detect traces of it in the CMB map, in primordial gravity waves recorded as fluctuations in patches of over density/underdensity in the oldest matter we can see. Hazardous longshots like that, that you have to be skeptical about. Anyway here's the link to QC research since 2009:
    http://inspirehep.net/search?ln=en&...2y=2013&sf=&so=a&rm=citation&rg=50&sc=0&of=hb
    You have to judge for yourself how you think things are going. at least these people are trying.
    I generally watch for work Abhay Asktekar (Penn State) and his group, and lately I have found the work by Jonathan Engle at Florida-Atlantic especially interesting.
    A french guy, Aurelien Barrau is especially good on the *observational testing* side. How to look for the imprint of the bounce in the CMB map.
    In case anyone is curious about the quality of talent getting into the field at grad student/postdoc level here is a sample of 20 minute VIDEO talks by young people in Lqc:
    http://pirsa.org/displayFlash.php?id=13070043
    Wilson-Ewing (Penn State PhD, co-author with Ashtekar), Julien Grain (coauthor with Barrau), Linda Linsefors (also in Barrau group), Andrea Dapor(Warsaw group)
     
    Last edited: Aug 31, 2013
  11. Sep 1, 2013 #10
    No Marcus. That is not correct. You speak with great authority. This is a science forum after all and in science we seek the truth and try to avoid letting petty human emotions get in the way.

    It is with great reluctance then that I disagree with you and by association I suppose most of the Cosmology community. I do not believe, on dynamic grounds and all that I see around me in Nature, that before the Big Bang there was necessarily a classical universe like ours in basic physics.
     
    Last edited: Sep 1, 2013
  12. Sep 1, 2013 #11

    marcus

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    It's fine to disagree! And simply not believing what that particular MODEL says does not put you in opposition to "most of the Cosmology community".

    Skepticism is an appropriate and respected attitude. I thought I made it clear that there are several different theoretical models that interest researchers (they are not all "bounce") and which are being studied and have people looking for ways to test them. I don't know of any consensus of BELIEF about models of the start of expansion.

    I can't read minds, I've talked with a number of researchers and no one ever said "I believe" in this or that picture of the start of expansion. My sense of these people is that what matters is which concepts you find plausible, interesting and worth exploring, not what you believe in. People wisely reserve judgment and wait for more evidence to accumulate. It's not what you believe in so much as what you think is promising enough to devote several years of your life working on.

    It seems to me that in listening to informal discussions I've heard more disbelief expressed than belief. Finding good reasons to RULE OUT one or another of the various alternative explanations is an important part of the process and I suppose ought to be encouraged.
     
    Last edited: Sep 1, 2013
  13. Sep 1, 2013 #12
    Marcus,
    If you're not an authority on cosmology, WHO IS?
     
  14. Sep 1, 2013 #13

    marcus

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    Thanks for the friendly words--I'm sure you're not dead serious. But suppose we were to ask that Who is? question literally, and let people answer.
    I have the impression that there are some real experts (not amateur cosmology-watchers like myself) around Physicsforums. I can't remember all the names and they don't check in all the time.
    Two people that come to mind are George Jones and Brian Powell (bapowell). You can probably find Brian Powell journal-type articles on arxiv.org.

    Cristo, the moderator, was a PhD student in astrophysics or cosmology last time I heard, may have gotten his degree already. There used to be a frequent poster here named SpaceTiger, who was a PhD student at Princeton, but he got his degree and moved on to junior faculty or something and got too busy. Haven't seen him for a while. There's Ben Crowell, and someone whose name has 3 letters beginning with M, like "MFB". I don't remember (bad memory for names)--real specialty is particle physics but seems to know cosmology as well. I could be wrong or getting people mixed up.

    Actually there's a bunch more, too many to list now that I think of it. I mean academically qualified people. Of course one of the things about PF (which I like) is ANONYMITY. You look at the quality of someone's posts and you gradually realize the person really knows what they're talking about and you guess, but often you never pick up any definite information. Maybe a chance reference in some thread.

    Maybe other people (if they see your question) will answer. Now that I reflect on it, I can think of other names that should be on the list if I was supposed to make it more complete. I'll let others complete the job and continue (if they want) where I'm leaving off.
     
    Last edited: Sep 1, 2013
  15. Sep 1, 2013 #14
    I think marcus is showing humility and, may I say, wisdom - which I think are very good traits for those who are interested in science and/or active scientists. And I would like to take the opportunity to quote Carl Sagan, if I may:

    (from the so-called Baloney Detection Kit, based on the book "The Demon Haunted World: Science as a candle in the dark")

    I think this is wisely put by Sagan.

    If I would allow myself to be bold and add something to Sagan's words I would say that in science, the final expert is Nature itself (or the Universe, if you like) - i.e. it is experiments/observations which ultimately decide which models hold and which do not.

    And oh, by the way, I might aswell point out that I am myself just a very interested amateur when it comes to cosmology. Marcus (and many others here) knows much more about the field than me. I know some basics, but I'm still trying to learn and catch up pieces here and there as I go along... particularly about new research and new models under development, which I know quite little about.
     
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